Magnetic field sensor for an access point

ABSTRACT

A device may monitor an entry to or exit from a space via an access point. The entry access point has a first component and a second component that are separable from each other to create an opening for the entry or exit. The device may comprise a first part for mounting to one of said components and which has a processing component and a sensing component for sensing in multiple dimensions a magnetic field emanating from a second part mounted on the other of the first and second components. The processing component is configured: to receive an indication of the sensed magnetic field; detect a change of condition at the access point when a dynamic quality of the indication of the sensed magnetic field satisfies a predefined criterion; and output an indication of the detected change of condition. There may be a corresponding system, method and memory.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/728,133, filed Dec. 27, 2019, which claims the benefit of priority ofUK Application Number 1821310.8, filed Dec. 31, 2018, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a magnetic field sensor for an accesspoint.

BACKGROUND

An entry point to an indoor or outdoor space may be provided via a door,gate or window. The entry point's state (open or closed) be may detectedby a device installed on the entry point, the device having a magneticpart and a magnetic field sensing part, the respective parts beinginstalled on different ones of an openable component (e.g.door/window/gate) of the entry point and a fixed component (e.g. adoor/window frame or a gate post) of the entry point. However, suchdevices are subjectable to tampering attempts. For example, an intrudermay place a magnetic of their own adjacent the magnetic sensor so thatthe magnetic sensor does not sense a magnetic field absence when theentry point is opened, and thus does not detect that the state of theentry point has changed from closed to opened. While various solutionshave been attempted to solve this and other tampering threats, therecontinues to be a need for solutions to such and other problems of theprior art, including susceptibility to other tampering methods, or toprovide a market alternative.

Reference to any prior art in this specification is not anacknowledgement or suggestion that this prior art forms part of thecommon general knowledge in any jurisdiction, or globally, or that thisprior art could reasonably be expected to be understood, regarded asrelevant/or combined with other pieces of prior art by a person skilledin the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual illustration of an exemplary sensor system of oneor more embodiments of the invention, installed on an exemplary accesspoint;

FIG. 2 is a block diagram showing principal components of a device thatis a part of the sensor system of FIG. 1 ;

FIG. 3 is a graphical representation of a component of an exemplarymagnetic field against time, sensed by the device of FIG. 2 ;

FIG. 4 is a graph showing a filtered output of the sensed magnetic fieldof FIG. 3 showing a dynamic component of the sensed magnetic field,compared with thresholds, according to one or more embodiments of theinvention;

FIG. 5 is a graph showing a low frequency portion of the dynamiccomponent of FIG. 4 , compared with thresholds, according to one or moreembodiments of the invention;

FIG. 6 is a graph showing a high frequency portion of the dynamiccomponent of FIG. 4 , compared with thresholds, according to one or moreembodiments of the invention;

FIG. 7 is a graph showing a differential of the sensed magnetic field ofFIG. 3 , compared with thresholds, according to one or more embodimentsof the invention;

FIG. 8 is a graph showing a low frequency portion of the differential ofFIG. 7 , compared with thresholds, according to one or more embodimentsof the invention;

FIG. 9 is a graph showing a high frequency portion of the differentialof FIG. 7 , compared with thresholds, according to one or moreembodiments of the invention;

FIG. 10 is a graph showing changes in the sensed magnetic field of FIG.3 over different durations of time, according to one or more embodimentsof the invention;

FIG. 11 is a graph showing the changes in the sensed magnetic field fromFIG. 10 for relatively short durations of time, compared withthresholds, according to one or more embodiments of the invention;

FIG. 12 is a graph showing the changes in the sensed magnetic field fromFIG. 10 for relatively long durations of time, compared with thresholds,according to one or more embodiments of the invention; and

FIG. 13 is a three-dimensional representation of a bias of a sensedmagnetic field and various thresholds used for comparison with thesensed magnetic field, according to one or more embodiments of theinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various embodiments of the invention are set out in the claims at theend of this specification. Further aspects of the present invention andfurther embodiments of the aspects described in the preceding paragraphswill become apparent from the appended figures and the followingdescription, given by way of non-limiting example only. As will beappreciated, other embodiments are also possible and are within thescope of the claims.

A first aspect of the present invention provides a device for monitoringan entry to or exit from a space via an access point, the entry accesspoint having a first component and a second component that are separablefrom each other to create an opening for the entry or exit, the devicecomprising:

a first part for mounting to one of first component and the secondcomponent, the first part having:

-   -   a sensing component for sensing in multiple dimensions a        magnetic field emanating from a second part mounted on the other        of the first component and the second component, and    -   a processing component configured to:        -   receive an indication of the sensed magnetic field;        -   detect a change of condition at the access point when a            dynamic quality of the indication of the sensed magnetic            field satisfies a predefined criterion; and        -   output an indication of the detected change of condition.

By detecting a change of condition/state based on a dynamic quality ofthe indication of the sensed magnetic field, the device operate mayoperate without having to calibrate for different or changedinstallations. In other words, the device may operate with indifferenceto the actual/absolute magnitude of the magnetic field received by thesensing component in any of the dimensions. Further, by sensing themagnetic field in multiple dimensions as opposed to a single dimensionthere may be, in some embodiments, even greater flexibility ininstallation by allowing different installation orientations of thefirst part with respect to the second part, and/or may allow for greatersensitivity in detecting a change of condition, as approaches of atamper magnet from any direction may be detected. In some embodiments,the multiple dimensions may more specifically be at least 3 dimensions,or more yet specifically be 3 dimensions in some embodiments. The 3dimensions is in some embodiments are 3 Cartesian dimensions.

It is also beneficial to minimize or reduce the number inconsequential(i.e. false alarm) output events from the device, as each output eventconsumes power. Thus, appropriate settings of various detectionthresholds is important and may benefit from being based on a dynamicaspect of the sensed magnetic field.

In some embodiments outputting an indication of the detected change ofcondition comprises instructing a transmission component to transmit anindication of the detected change of condition, for example to a controlpanel, which may optionally treat the indication as an alert. Outputtingan indication of the detected change of condition may additionally oralternatively comprise an audible and/or visible output such as an alarmsound and/or a light emission.

In some embodiments, detecting that a dynamic quality satisfies thepredefined criterion comprises: comparing a first value representing anamount or rate of a relatively slower magnetic field change with a firstthreshold and comparing a second value representing an amount or rate ofa relatively quicker magnetic field change with a second threshold, anda dynamic quality is detected as satisfying the predefined criterionwhen at least one of the first value is greater than the first thresholdand the second value is greater than the second threshold. In thecontext of thresholds as used herein, the term ‘greater’ is to beunderstood to mean having a greater magnitude than the threshold, so forexample, a negative change would need to be more negative than anegative threshold to be greater than the threshold. Also, as usedherein, ‘slow’, ‘relatively slower’ or ‘relatively slow’ and ‘quick’,‘relatively quicker’ or ‘relatively quick’ may be understood to beslower and quicker relative to each other. Advantageously, bothrelatively slow and relatively quick approaches of a tamper magnet maybe detected, making it difficult for a potential intruder to avoidtamper detection.

In some embodiments, detecting that a dynamic quality satisfies thepredefined criterion comprises a first comparison, with respect to afirst threshold, of a first dynamic quality that represents a firstmeasure of change with respect to time.

In some embodiments, the first measure of change with respect to time isan amount of change of the sensed magnetic field over a first durationof time.

In some embodiments, first measure of change with respect to time is arate or amount of change of a first spectral portion of the indicationof the magnetic field in a frequency domain. As will be appreciated themagnetic field need not actually be physically represented in afrequency domain (by a processor) in order to calculate the firstmeasure of change.

The first spectral portion of the magnetic field in a frequency domainmay, for example, be derived from a filter that acts as at least one ofa high pass (eg. AC coupled) or and a band pass filter.

In some embodiments, the first spectral portion of the magnetic field ina frequency domain may be derived digitally, e.g. based on one or morecomponents of a Fourier transform of the indication of the sensedmagnetic field, or by an analog circuit, e.g, based on an output of adifferentiating amplifier.

In some embodiments, detecting that a dynamic quality satisfies thepredefined criterion comprises a second comparison, with respect to asecond threshold, of a second dynamic quality that represents an secondmeasure of change with respect to time.

In some embodiments the first measure of change with respect to time isan amount of change of the sensed magnetic field over a first durationof time; and the second measure of change with respect to time is anamount of change of the sensed magnetic field over a second duration oftime that is different to the first duration of time.

Optionally, the amount of change of the sensed magnetic field over thesecond duration of time may be represented as an accumulated change overthe second duration of time or as an average change over a plurality ofinstances of a smaller duration of time within the second duration oftime. The smaller duration of time may, for example, the first durationof time.

In some embodiments, second measure of change with respect to time is arate or amount of change of a second spectral portion of the indicationof the magnetic field in a frequency domain, wherein the first spectralportion of the magnetic field in a frequency domain includes a lowerfrequency portion of the magnetic field than the second spectralportion.

For example, the second spectral portion of the magnetic field in afrequency domain may be derived from a filter that acts as at least oneof a high pass or and a band pass filter having a lower frequency cutoffthat is at a higher frequency cutoff of the first spectral portion.

In some embodiments, the first and second thresholds are different. Insome embodiments, the first and second thresholds are the same.

In some embodiments the predefined condition is that any one of thecomparisons passes a test, for example that the representation of theamount of change of the sensed magnetic field has is greater than thethreshold (e.g, the first or second threshold, whichever is relevant).

Preferably the sensing component is a solid-state magnetometer, forexample a hall-effect based sensor, or any other solid-statemagnetometer.

In some embodiments, the at least one dynamic quality of the sensemagnetic field is a dynamic quality in any one of the multipledimensions.

In some embodiments, the at least one dynamic quality of the sensemagnetic field is a dynamic quality in more than one of the multipledimensions.

In some embodiments, the at least one dynamic quality of the sensemagnetic field is a dynamic quality in all of the multiple dimensions.

In some embodiments, the multiple dimensions comprise three dimensions.

In some embodiments, the multiple dimensions consist of threedimensions.

Conveniently, in some embodiments, the multiple dimensions are cartesiandimensions represented by cartesian coordinate system. Thus, thethresholds, may be thresholds on x, y, z axes. In other embodiments, themultiple dimensions may be represented by a spherical or cylindricalcoordinate system, with the thresholds in the corresponding axes ofthose coordinate systems.

A second aspect of the present invention provides a sensor system fordetecting an intrusion attempt at an entry point to a space, the systemcomprising: the device the first aspect of the present invention; andsaid second part.

A third aspect of the present invention provides a method for monitoringan entry to or exit from a space via an access point, the entry accesspoint having a first component and a second component that are separablefrom each other to create an opening for the entry or exit, wherein afirst part is mounting to one of the first component and the secondcomponent, the first part having a sensing component for sensing inmultiple dimensions a magnetic field emanating from a second partmounted on the other of the first component and the second component,wherein the method comprises:

receiving an indication of the sensed magnetic field;

detecting a change of condition at the access point when a dynamicquality of the indication of the sensed magnetic field satisfies apredefined criterion; and

outputting an indication of the detected change of condition.

As will be appreciated, each step executed by the device of the firstaspect of the present invention may also be steps of the third aspect ofthe present invention.

A fourth aspect of the present invention provides a non-transient memoryfor storing code for execution by a processing system wherein uponexecuting the code the processing system is configured to perform themethod of the third aspect of invention.

An exemplary sensor system 100 in accordance with an embodiment of thepresent invention is depicted schematically in FIG. 1 , showing thesensor system 100 installed at an access point 102 to a space 104. Thesensor system 100 comprises a device 106 which is a first part of thesensor system 100 and a physically separate part 108 which is a secondpart of the sensor system 100. In this example, the first part 106 isinstalled on a first component 110 of the access point 102, at alocation 112 adjacent the second part 108, which is installed on asecond component 114 of the access point 102, the first and secondcomponents being separable from each other to create an opening forentering or exiting the space 104 via the access point 102. In theillustrated embodiment the first component is a door, and the secondcomponent is a door frame, but in other embodiments the first componentmay be the door frame and second component may be the door, or theaccess point may be a window access point with the first and secondcomponents being a window and a window frame or a gate access point withthe first and second components being a gate and either a gate post oranother gate, for example. In the illustrated example the access pointopens by the first component sliding in a direction 116 away from thesecond component, resulting in a corresponding movement of the firstpart 106 away from the second part 108. In this example the movement ofthe first component 110 away from the second component 114 is in asingle linear axis, which is in the an ‘x’ axis in the figure. However,in other embodiments, the other directions of movement may be involved.For example, the first components 110 may be a hinged component, whichrotated on the hinge to open the access point 102, for example resultinginitially in a movement perpendicular the page (along the ‘z’ axis)before movement along the x axis becomes noticeable. As will beappreciated, other opening paths are possible for other access pointconfigurations to which the present invention may be applied. Forexample, some access points configured to be opened by moving either oneor both of the first or second components.

The first part of the system 100, i.e. the device 106, senses a magneticfield in 3 dimensions, e.g. the x, y and z dimensions, at itsinstallation location 112. The second part 108 of the system 100comprises a magnet that emanates a magnetic field with magnetic northand south poles, marked N and S in the figure, which is sensed by thedevice 106 when the access point 102 is closed enough for the magneticfield from the second part 108 to be fall within the sensitivitythreshold of the device 106. When the access point is sufficiently openthe sensed magnetic field drops below a threshold value (the thresholdvalue being greater than the sensitivity threshold), the device 100 maydetermine that the access point is open, or at least that a change ofstate (i.e, condition) of has occurred.

A potential intruder may attempt to tamper with the system 100 to avoiddetection by placing a tamper magnet adjacent the device 106 in the sameor similar relative disposition to the device 106 as the second part108. In doing so, the intention of the intruder is to keep the magneticfield sensed by the device 106 above the threshold value, while theaccess point 102 is opened, so that no door open event is detected.

Such a tamper attempt may however result in an increase in magneticfield at the device 106 which could be detected by the device to therebyidentify that a magnetic tamper has, or may have, occurred. One optionto have sensitive detection of the door no longer being in the closed,non-tampered state is to calibrate the device 106 to place relativelyclose thresholds above and below the ‘normal’ magnetic field magnitude,i.e. the field that exists in the non-tampered closed-access pointstate. However, such calibration may be laborious or done incorrectly bythe installer. Further, the normal magnetic field may change with time,for example due to deterioration of how well the two components 110 and114 of the access point 102 fit together when the access point 102 isclosed.

However, the device 106 is provided with some ability to detect a changeof state of the magnetic field at the location 112 of the device 106with indifference to actual magnetic of the magnetic field in the normal(door closed non-tamper) state by detecting a change of condition at theaccess point based on a dynamic quality of the sensed magnetic field.Further, the exemplary device 106 senses the magnetic field in 3dimensions to detect a change of condition when a tamper magneticapproaches the device 106 from any direction. Further, the dynamicquality of the magnetic field may be simultaneously assessed in aplurality of ways to identify relatively slow magnetic field changes andrelatively quick magnetic field changes, distinctly from each other, andoptionally identify one or more speeds of change in-between these two(fast and slow) extremes.

A block diagram showing an example of components that may be used forthe device 106 is illustrated in FIG. 2 . The device 106 has a housing200 with an adhesive for mounting the housing to one of the componentsof the access point. Inside the housing 200 are a sensing component 202,a processing component 204 a memory component 206 and an outputtingcomponent 208. The term ‘component’ in the context may be one device, apart or one device, or a plurality of devices. Thus, in someembodiments, one or more of the components 202-208 may be integratedonto a common device, for an example an integrated circuit. In any case,the processing component 204 may be comprised of one or more processingchips. The processing component 204 may include one or more processingdevices. For example, the one or more processing devices may comprise:control circuitry; and/or processor circuitry; and/or at least oneapplication specific integrated circuit (ASIC); and/or at least onefield programmable gate array (FPGA); and/or single or multi-processorarchitectures; and/or sequential/parallel architectures; and/or at leastone programmable logic controllers (PLCs); and/or at least onemicroprocessor; and/or at least one microcontroller; and/or a centralprocessing unit (CPU); and/or a graphics processing unit (GPU), and/ortransceiver(s) to perform the methods.

At least one memory component 206 may be separate from the processor(s)and/or partly or wholly integrated onto a common chip(s) with theprocessor(s). The at least one memory may store code that, when read bythe processor(s), causes performance of any of the methods describedherein, and/or as illustrated in in the drawings. For example, thememory may comprise: volatile memory, for example, one or more dynamicrandom access (DRAM) modules and/or static random access memory (SRAM)modules; and/or non-volatile memory, for example, one or more read onlymemory (ROM) modules; which for example may comprise a Flash memoryand/or other electrically erasable programmable read-only memory(EEPROM) device. The code may for example be software, firmware, orhardware description language (HDL) or may be any combination of theseor any other form of code for one or more processors that is known by aperson skilled in the art.

The device is also powered by a battery 210 held within the housing 200of the device.

The processing component 204 may have a Central Processing Unit (CPU)122 for performing high level control of the operation of the device 106and interfacing the memory component 204, the output component 208, anda dynamic feature extraction component 214 having analog and digitalcircuitry for receiving an indication of a sensed magnetic field fromthe sensing component 202. The CPU may, in some embodiments, alsoreceive the raw indication of the sensed magnetic field from the sensingcomponent 202. The processing component 204 may instruct the outputcomponent 208, which may comprise a transceiver, to wirelessly transmitdata, for example data identifying a detected change of condition of thesensed magnetic field, to a control panel (not shown). The transceivermay also act as an input component for updating the code in the memorycomponent 206, from a remote storage of the code, e.g. via the controlpanel or a remote server. The output component may also include aspeaker and/or visual indicator(s) e.g. LED(s) to provide an audioand/or visual indication of a detected change of condition of the sensedmagnetic field.

The sensing component 202 may be a solid-state magnetometer for sensingmagnetic field in three dimensions. The magnetometer may be a singledevice or mar be comprised of a plurality of devices configured to sensemagnetic fields in orthogonal directions. In any case, the sensingcomponent 202 may output an indication of the sensed magnetic field asmagnitudes, proportional to magnetic field strength or intensity, in therespective dimensions.

The processing component 204 may include, between the sensing component202 and a central processing unit (CPU) 212, or integrated into a CPU, adynamic feature extraction component 214 for extracting one or moredynamic qualities of the indication of the sensed magnetic fieldreceived from the sensing component 202. The dynamic feature extractioncomponent may extract relatively slow magnetic field changes distinctlyfrom relatively quick magnetic field changes to assess separately todetect slow and fast magnetic field changes.

More specifically in some embodiments, the dynamic feature extractioncomponent 214 may include an analog circuit and/or digital processing toextract the dynamic quality by AC coupling the sensed magnetic field. Anexample is discussed herein in relation to FIG. 4 . As will beunderstood, the AC coupling is effectively a high pass filter. However,the dynamic feature extraction component 214 may include a plurality offilters and/or may provide band pass filtering (i.e. in effect,combining the low pass filter with a high pass filter) to extractvarious portions of the frequency spectrum, e.g. a low and highfrequency portion of the spectrum, for example as will be discussedherein in relation to FIGS. 5 and 6 , respectively. The low and highfrequency component extraction may be provided for example bydifferentiating amplifier circuits, or may be done digitally (by aFourier transform or other known methods), or any other way known in theart. Optionally a change of condition may be determined to have occurredwhen an amount of change of a frequency component is greater than athreshold.

In other embodiments, the extracted dynamic component may be a rate ofchange of a dynamic component of the magnetic field. For example, thesensed magnetic field may be differentiated (e.g. as will be discussedin relation to FIG. 7 ), optionally with low and frequency componentsseparated from each other (FIGS. 8 and 9 ). For example, in an analogrealm this may involve the use of differentiating amplifiers configuredfor extracting different components (portions) of frequency spectrum.

In any case, in some embodiments, an extracted portion of the frequencyspectrum may have a lower end of a frequency range at or below 100millihertz (for example a low frequency cut-off at 100 millihertz). Inother embodiments, where the extracted portion of the frequency spectrumhas a relatively low frequency portion and a relative high frequencyportion, the relatively low frequency portion may range from at or below100 millihertz to between 1 and 10 about Hz, for example the range maybe from 100 millihertz to 10 Hz. The relatively high frequency portionmay have a lower end of a frequency range that corresponds to (e.g.matches) an upper end of a frequency range of the relatively lowfrequency portion. For example, the lower end of the relatively highfrequency portion may be between 1 and 10 hertz, and in some embodimentsmore specifically 10 hertz. An upper end of the high frequency portionmay be defined by a sampling rate of the magnetic sensor or controller.In some embodiments the upper end may be 100 to 200 hertz, e.g. 100hertz in some embodiments.

In other embodiments, the dynamic feature extraction component 214 mayinclude a plurality of sample and hold circuits sampling at differentfrequencies which may be used by the CPU to identify changes in magneticfield over different durations of time, as will be discussed in relationto FIG. 10 to FIG. 12 . As an alternative to using different samplingfrequencies, an output of an analog to digital converter may beresampled, in the dynamic feature extraction component 214 (especiallyin some digitally implemented embodiments), at a slower rate to identifychanges in magnetic field over a longer duration of time. The shorterand longer durations of time with which magnetic field changes areassessed may be have the same starting time or may have differentstarting times. The durations of time may be predefined. Further changesmay be assessed continuously using successive contiguous windows (byanalyzing a series of X millisecond windows that start every Xmilliseconds) or a sliding window (by analyzing a series of Xmillisecond windows that start every X/Y milliseconds, where Y isgreater than 1). As will be appreciated the dynamic qualities of thesensed magnetic field may be extracted and/or characterized in any otherway known to the person skilled in the art, and this may be done foranalysis of relatively slow magnetic field changes distinctly fromrelatively quick magnetic field changes, and optionally one or morespeeds of change in-between these two extremes.

The extracted features are compared with various thresholds by the CPU212 or some other processing component prior to processing by the CPU212. In some embodiments, for each dimension, a relatively low frequencycomponent of the sensed magnet field is continuously compared with apositive and a negative threshold, which may be the same or different toeach other, and in some embodiments, a relatively high frequencycomponent of the sensed magnet field is continuously compared with apositive and a negative threshold, which may be the same or different toeach other, and may be the same or different to the thresholds used forthe relatively low frequency component. An indication of a change ofstate is outputted by output component 208 when any one of the extractedfrequency components has a value outside of the bounds set by thepositive and lower thresholds. An example of such an embodiment isdepicted in FIGS. 5 and 6 . In other embodiments, the rate of change ofthe extracted frequency component(s) may be compared with the thresholdsto determine whether a change of condition has occurred. Examples ofsuch an embodiment is illustrated in FIGS. 8 and 9 , and in FIGS. 10 to12 , the latter of which depicts an amount of change of variousdurations of time. The CPU 204 may be configured to indicate a change ofcondition when such a change of condition is detected in any one ofmeasured the dimensions.

FIGS. 3 to 10 will now be discussed in greater detail. FIG. 3illustrates an exemplary sensed magnetic field over a period of time, inan x direction. The magnetic field in this example may be either the allof the magnetic field (if is all in the x direction) or may be acomponent of the magnetic that is in the x direction. In FIG. 4 thefeature extraction component 214 is AC coupled, or in any other knownway filtered to remove the sensed magnetic bias. The processingcomponent 204 then compare the filtered output to positive and negativethresholds 402 and 404 respectively to identify when the filteredmagnetic field is greater than either of the components, which in thisexample occurs during time intervals 406 and 408, respectively. A changeof state may be indicated to identify the events corresponding to thetime intervals 406 and 408, and optionally may transmit the duration ofthe respective time intervals 406 and 408 as further information.

In the exemplary embodiment depicted in FIGS. 5 and 6 , the dynamiccomponent is separated, before or after the magnetic bias removal, intolow and high frequency components, respectively. Each of the frequencycomponents is compared with positive thresholds 502, 602 and negativethresholds 504, 604. The positive threshold for a given frequencycomponent may be of the same or different magnitude to the negativethreshold for the same frequency component. Further the thresholds maybe different for the different frequency components to allow fordifferent sensitivities to different frequencies of magnetic change.

Though it may be considered that measuring a magnitude of a frequencycomponent of a changing magnitude field is analogous with assessing arate of change of the magnetic field, FIGS. 7 to 10 illustrateembodiments in which an actual rate of change of the magnetic field isdirectly assessed. In FIG. 7 shows, in the bottom of the figure, aconceptual/approximate representation of a rate of change of a magneticfield shown in the top of the picture, which is a reproduction of themagnetic field shown in FIG. 3 .

The differentiated magnetic field (i.e. its rate of change), d(B(x))/dt,is compared with positive and negative thresholds 702 and 704respectively. As a result, time periods 706, 708 and 710 are detected aschange of magnetic states.

FIGS. 8 and 9 show the differentiated magnetic field of FIG. 7 separatedinto low and high frequency components, respectively. Each of thefrequency components is compared with positive thresholds 802, 902 andnegative thresholds 804, 904. The positive threshold for a givenfrequency component may be of the same or different magnitude to thenegative threshold for the same frequency component. Further thethresholds may be different for the different frequency components toallow for different detection sensitivities for different frequencies ofmagnetic change.

In another embodiment, illustrated in FIG. 10 , rates of change are alsoassessed, but rather than by performing continuous differentiating ofthe magnetic field, the identification of relatively slow and relativelyfast rates of change are determined discretely by comparing amounts ofmagnetic field change over relatively long and relatively shortdurations, respectively. In the figure, a first, relatively short timeduration occurs at time interval 1002, and at contiguous times beforeand afterwards, such as at time interval 1004, and at a later time 1006.The amount of change of the sensed magnetic field over times 1002 and1006 are indicated by the amounts 1008 and 1010, respectively. Theamount of the change 1008 and 1010 with respect to the short durationchanges in time is shown in FIG. 11 in comparison with positive andnegative thresholds 1102 and 1104, respectively. The change 1008 is apositive change but is less than the positive threshold 1102, whereasthe change 1010 is a negative change that is greater (in terms ofmagnitude) than the negative threshold 1104. As a result, a change ofstate is only indicated for the change 1010.

In this exemplary embodiment, changes in the sensed magnetic field arealso be assessed over longer time durations, such as during timeinterval 1012, 1014 and 1016, to give an indication of slower changes inmagnetic field. The changes in magnetic field corresponding to intervals1012 and 1016 are indicated by changes 1022 and 1024, respectively. FIG.12 shows the changes 1022 and 1024 in comparison with positive andnegative thresholds 1202 and 1204, respectively, for the longer durationmeasurements. In this example, only the change 1202 is greater than thethreshold, so only that change may result in an indication of a changeof state.

Optionally in the embodiment of FIGS. 10 to 12 , the outputs of thecomparisons with the threshold may be processed to require a predefinedplurality of successive comparator outputs (i.e. over successive timeintervals) to be the same, in order to reduce noise and/or misleadingresults. For example, outputting an indication of a change of state mayrequire a predefined plurality (e.g. 2 in some embodiments) ofsuccessive changes that are greater than the positive threshold or aplurality of successive changes greater (in magnitude) than the negativethreshold.

In some embodiments, the processing component is further configured toalso detect a change of condition at the access point when theindication of the sensed magnetic field is greater than any one of aplurality of thresholds, wherein a plurality of said thresholds lie indifferent axes that are orthogonal to each other.

Thus, regardless of whether a potential intruder is able to evadedetection of a tamper magnet based on a dynamic quality of the sensedmagnetic field (for example by moving the magnet with a speed outside arange of detectable speeds), the processing component is able to detecta change in state based on an increase in the absolute value of thesensed magnetic field. Such detection may, however, optionally usepre-set thresholds rather than thresholds set at installation. Althoughgreater sensitivity may be achieved by calibrating the thresholds for aparticular installation, sensitivity to a tamper magnet may nonethelessbe achieved with pre-set thresholds due to the use of comparisonthresholds in a plurality of orthogonal axes, and/or due to thepre-set-thresholds only being needed to provide a form of redundancy orback-up should a potential intruder be able to avoid sensing of thedynamic quality of the magnetic field.

In many installations, however, a vast majority of the sensed magneticfield will lie in one direction of one the axes, which may be pre-knownor determined by a measurement at installation. Thus, in someembodiments, a larger threshold may advantageously be used for one ofthe axes than the other axes. In some embodiments, the larger thresholdmay be used for only one direction of said one of the axes. Optionallythe threshold in an opposite direction to the one direction may be zero.For example, if the majority of the magnetic field is known to lie inthe +X axis, then any negative value for X may be determined as a changeof state.

In some embodiments, the plurality of thresholds comprises sixthresholds, wherein said plurality of said thresholds lie in threedifferent axes that are orthogonal to each other. Thus, there may forexample by thresholds in +x, −x, +y, y, +z or −z axes, one of which mayoptionally be zero.

For example, turning to FIG. 13 , the bias of a sensed magnetic field,i.e. the field in a non-tampered closed accessed point, may threedimensional coordinates Xb, Yb, Zb. However, Xb is much larger than Yband Zb. Yb and Zb are relatively small such that they are less thanpre-defined thresholds +Yth, −Yth and +Zth, −Zth, respectively. Afactory set threshold may be used for +Xth, For example +Xth can be setto a measured value of the magnetic field when the first part 106 andsecond part 108 are touching with the magnetic field aligned in the +Xdirection, since in use the first part 106 and second part may beinstalled with at least some separation between them and therefore alower magnetic field than the factory measurement. Optionally +Xth mayin any case be slightly larger than that measured value. The thresholdfor −Xth may be zero or a non-zero value.

Nonetheless, the present invention is still advantageous even if iscalibration is performed to set one or more of +Xth, −Xth, +Yth, −Yth,+Zth, or −Zth, which is optional in some embodiments, as at leastbecause detecting of a dynamic quality of the sensed magnetic fieldenables some adaptation to any reductions in the bias magnetic fieldover time, and in any case provides another mode of detectingmagnetically significant events.

The meaning of “first” and “second”, as used herein, is not intended toimply a temporal ordering in which the first must precede the second.

Where a given item is referenced herein with the preposition “a” or“an”, it is not intended to exclude the possibility of additionalinstances of such an item, unless context requires otherwise.

Where the specification defines a range, the stated outer extremities ofthe range are part of the range, unless context requires exclusion ofthe outer extremities from the range. For example, a range defined interms of being between X and Y or from X to Y, should be interpreted asincluding X and Y.

The present invention also provides a storage medium storing processorimplementable code, which, when executed by a processing system,implement the processes of any of the embodiments described above.

In one embodiment, the storage medium, can comprise a non-transientstorage medium storing code for execution by a processor of a machine tocarry out the method. Embodiments can be implemented in programmabledigital logic that implements computer code. The code can be supplied tothe programmable logic, such as a processor, microprocessor orprocessing system, on a carrier medium. One embodiment is anon-transitory storage medium that stores the code, such as asolid-state memory, magnetic media (hard disk drive), or optical media(Compact disc (CD) or digital versatile disc (DVD)).

As used herein, except where the context requires otherwise, the terms“comprises”, “includes”, “has”, and grammatical variants of these terms,are not intended to be exhaustive. They are intended to allow for thepossibility of further additives, components, integers or steps.

The invention disclosed and defined herein extends to all plausiblecombinations of two or more of the individual features mentioned orevident from the text or drawings. All of these different combinationsconstitute various alternative aspects of the invention.

The invention claimed is:
 1. A device for monitoring an entry to or exitfrom a space via an access point, the access point having a firstcomponent and a second component that are separable from each other tocreate an opening for the entry or exit, the device comprising: a firstpart for mounting to one of first component and the second component,the first part having: a sensing component for sensing in multipledimensions a magnetic field emanating from a second part mounted on theother of the first component and the second component; and a processingcomponent configured to: receive an indication of the sensed magneticfield; detect a change of condition at the access point when a dynamicquality of the indication of the sensed magnetic field satisfies apredefined criterion; and output an indication of the detected change ofcondition; wherein the processing component is configured to detect achange of condition at the access point, by: performing a firstcomparison involving a first value representing a first rate of changethat represents an amount of change of the sensed magnetic field over afirst duration of time, the first comparison being of the first valuewith respect to a first threshold; performing a second comparisoninvolving a second value representing a second rate of change thatrepresents an amount of change of the sensed magnetic field over asecond duration of time that is different to the first duration of time,the second comparison being of the second value with respect to a secondthreshold; and detecting a change of condition at the access point whenat least one of the first value is greater than the first threshold andthe second value is greater than the second threshold.
 2. The deviceaccording to claim 1, wherein the first and second thresholds aredifferent from each other.
 3. The device according to claim 1, whereinthe first threshold has a same magnitude as the second threshold.
 4. Thedevice according to claim 1, wherein the sensing component is asolid-state magnetometer, wherein the indication of the magnetic fieldis in multiple dimensions.
 5. The device according to claim 1, whereinthe sensing component is a solid-state magnetometer, wherein theindication of the magnetic field comprises a three-dimensionalrepresentation of the sensed magnetic field.
 6. The device according toclaim 1, wherein the processing component is further configured to alsodetect a change of condition at the access point when the indication ofthe sensed magnetic field is greater than any of the first and secondthresholds, wherein said first and second thresholds lie in differentaxes that are orthogonal to each other.
 7. The device according to claim6, wherein one of said thresholds has a larger magnitude than the otherof said thresholds.
 8. A method for monitoring an entry to or exit froma space via an access point, the access point having a first componentand a second component that are separable from each other to create anopening for the entry or exit, wherein a first part is mounted to one ofthe first component and the second component, the first part having asensing component for sensing in multiple dimensions a magnetic fieldemanating from a second part mounted on the other of the first componentand the second component, wherein the method comprises: receiving anindication of the sensed magnetic field; detecting a change of conditionat the access point when a dynamic quality of the indication of thesensed magnetic field satisfies a predefined criterion; and outputtingan indication of the detected change of condition; wherein detecting achange of condition at the access point comprises: performing a firstcomparison involving a first value representing a first rate of changethat represents an amount of change of the sensed magnetic field over afirst duration of time, the first comparison being of the first valuewith respect to a first threshold; performing a second comparisoninvolving a second value representing a second rate of change thatrepresents an amount of change of the sensed magnetic field over asecond duration of time that is different to the first duration of time,the second comparison being of the second value with respect to a secondthreshold; and detecting a change of condition at the access point whenat least one of the first value is greater than the first threshold andthe second value is greater than the second threshold.
 9. The methodaccording to claim 8, wherein the first and second thresholds aredifferent to each other.
 10. The method according to claim 8, whereinthe first threshold has a same magnitude as the second threshold. 11.The method according to claim 8, wherein the sensing component is asolid-state magnetometer, wherein the indication of the magnetic fieldis in multiple dimensions.
 12. The method according to claim 8, whereinthe sensing component is a solid-state magnetometer, wherein theindication of the magnetic field comprises a three-dimensionalrepresentation of the sensed magnetic field.
 13. The method according toclaim 8, further comprising: detecting a change of condition at theaccess point when the indication of the sensed magnetic field is greaterthan any of the first and second thresholds, wherein said first andsecond thresholds lie in different axes that are orthogonal to eachother.
 14. The method according to claim 13, wherein one of saidthresholds has a larger magnitude than the other of said thresholds. 15.A carrier medium carrying code for execution by a processing system,wherein upon executing the code, the processing system is configured toperform operations comprising: monitoring an entry to or exit from aspace via an access point, the access point having a first component anda second component that are separable from each other to create anopening for the entry or exit, wherein a first part is mounted to one ofthe first component and the second component, the first part having asensing component for sensing in multiple dimensions a magnetic fieldemanating from a second part mounted on the other of the first componentand the second component, wherein monitoring the entry to or exit fromthe space via the access point comprises: receiving an indication of thesensed magnetic field; detecting a change of condition at the accesspoint when a dynamic quality of the indication of the sensed magneticfield satisfies a predefined criterion; and outputting an indication ofthe detected change of condition; wherein detecting a change ofcondition at the access point comprises: performing a first comparisoninvolving a first value representing a first rate of change thatrepresents an amount of change of the sensed magnetic field over a firstduration of time, the first comparison being of the first value withrespect to a first threshold; performing a second comparison involving asecond value representing a second rate of change that represents anamount of change of the sensed magnetic field over a second duration oftime that is different to the first duration of time, the secondcomparison being of the second value with respect to a second threshold;and detecting a change of condition at the access point when at leastone of the first value is greater than the first threshold and thesecond value is greater than the second threshold.
 16. The carriermedium according to claim 15, wherein the first and second thresholdsare different to each other.
 17. The carrier medium according to claim15, wherein the first threshold has a same magnitude as the secondthreshold.
 18. The carrier medium according to claim 15, wherein thesensing component is a solid-state magnetometer, wherein the indicationof the magnetic field is in multiple dimensions.
 19. The carrier mediumaccording to claim 15, wherein the sensing component is a solid-statemagnetometer, wherein the indication of the magnetic field comprises athree-dimensional representation of the sensed magnetic field.
 20. Thecarrier medium according to claim 19, wherein the operations furthercomprise: detecting a change of condition at the access point when theindication of the sensed magnetic field is greater than any of the firstand second thresholds, wherein said first and second thresholds lie indifferent axes that are orthogonal to each other, and wherein one ofsaid thresholds has a larger magnitude than the other of saidthresholds.